Question

Acid rain contains sulfuric acid and can cause lakes to become acidic. Acidic lakes may be treated with powdered limestone, impure CaCO₃, to neutralise the acidity forming calcium sulfate. If large lumps of limestone are used, instead of the powder, the reaction starts but soon stops, leaving most of the limestone unreacted. Which statement explains why the reaction starts but soon stops?

A) A layer of insoluble calcium sulfate forms on the surface of the lumps.
B) Limestone only contains small amounts of calcium carbonate.
C) Powdered limestone is more reactive than lumps of limestone.
D) The acid reacts with the calcium sulfate instead of the calcium carbonate.

Focus

In this video, we will go through the general rules of solubility for common salts to include nitrates, chlorides (including silver and lead), sulfates (including barium, calcium and lead), carbonates, hydroxides, salts of Group I cations and ammonium salts. We will also revise the chemical reaction an acid can undergo- with metals, bases and carbonates.

Acid Reaction

At O levels, we learn 3 different chemical reaction an acid can undergo.

Acids react with metals to produce salt and hydrogen gas.
Acids react with bases to produce salt and water.
Acids react with carbonates to produce salt, water and carbon dioxide gas.

In this question, we focus on the reaction of acid with carbonate. The salt formed in this reaction is calcium sulfate, which is interesting.

Salt Solublity

At O levels, we are to remember the solubility of different salts – nitrates, chlorides, sulfates, carbonates.
For sulfate salt, which is the product from this reaction, is mostly soluble, except calcium sulfate, lead(II) sulfate and barium sulfate. Hence, the salt formed in this reaction is actually an insoluble salt. This insoluble salt forms a layer on the surface of the solid reactant, resulting in the phenomenon observed – reaction proceeds but soon stops.

GCE O level, Singapore, 2014/P1/Q29

Topic: Acids and Bases, Salts, O Level Chemistry, Singapore

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